Nuclear fuel rod transfer canister having corrugated funnel

ABSTRACT

A canister for the consolidation of nuclear fuel rods (22) comprises an elongated, tapered box (24) of generally rectangular cross-section having upper (42) and lower (44) internal sections, the upper section having rectangular grid structure (46) and the lower section having vertically extending plate members (48). The upper section receives individual fuel rods in a relatively loosely packed rectangular array of rows and columns and urges the rods closer together as they are lowered into the box, such that the rods emerge through the lower end of the grid section in a relatively tightly rectangular array. The number of plate members (48) in the lower section corresponds to the number of rows of fuel rods in the upper section. Each plate has tapered side edges (204) abutting the tapered box, and a plurality of integrally formed, corrugated channels (206) corresponding to the number of rows of fuel rods. These channels guide the fuel rods into a triangularly pitched array, and the decreasing space between adjacent plates causes the fuel rods to move closer together as they move through the box. At the lower edge (202) of each plate the corrugations form a periodic series of alternating convex (208) and concave (210) arches such that each concave arch on one plate faces a concave arch on the adjacent plate. At their closest spacing, the edges of adjacent arches are separated by a distance substantially equal to a fuel rod diameter.

BACKGROUND OF THE INVENTION

The present invention relates to the storage of spent nuclear fuel, andin particular to apparatus employed to consolidate fuel rods afterremoval from the fuel assembly frame structure.

For a variety of reasons, the disposal, reprocessing or storage of spentnuclear fuel assemblies has posed significant obstacles to manyutilities operating nuclear power plants. As a consequence, efforts haverecently been directed toward consolidated nuclear fuel storage, bywhich the individual fuel rods are removed from spent fuel assembliesand stored in a tightly packed array until ultimate disposal methods canbe found.

In co-pending U.S. application Ser. No. 535,105, "System and Method forConsolidating Spent Nuclear Fuel", filed Sept. 23, 1983 and assigned toCombustion Engineering, Inc., apparatus is disclosed for removing fuelrods from two fuel assemblies and consolidating the rods into an areaequivalent to one fuel assembly. The consolidation is implemented atthree stations, one of which includes an interim fuel transfer canisterwhere the fuel rods are rearranged from a relatively loosely packedrectangular array into a relatively tightly packed triangular array.This is accomplished by inserting the rods between long flat plates,portions of which are contained within a funneled canister having atilted base. The canister walls, variation in plate lengths, and tiltedbase, tend to force the fuel rods into the desired final array.

It has been found that the transfer canister described in the foregoingapplication does not provide sufficient guidance to individual fuel rodsto permit the degree of trouble-free canister loading that is desired.Accordingly, a need was recognized to improve upon this prior canisterdesign.

SUMMARY OF THE INVENTION

According to the present invention, a canister for the consolidation ofnuclear fuel rods comprises an elongated, tapered box of generallyrectangular cross-section having upper and lower internal sections, theupper section having rectangular grid structure and the lower sectionhaving vertically extending plate members. The upper section receivesindividual fuel rods in a relatively loosely packed rectangular array ofrows and columns and converges and urges the rods closer together asthey are lowered into the box, such that the rods emerge through thelower end of the grid section in a relatively tightly rectangular array.The number of plate members in the lower section corresponds to thenumber of rows of fuel rods in the upper section. Each plate has taperedside edges abutting the tapered box, and a plurality of integrallyformed, corrugated channels corresponding to the number of rows of fuelrods. These channels guide the fuel rods into a triangularly pitchedarray, and the decreasing space between adjacent plates causes the fuelrods to move closer together and converge as they move through the box.At the lower edge of each plate the corrugations form a periodic seriesof alternating convex and concave arches such that each concave arch onone plate faces a concave arch on the adjacent plate. At their closestspacing, the edges of adjacent arches are separated by a distancesubstantially equal to a fuel rod diameter.

With the present invention, the fuel rods are more tightly constrainedduring their movement within the canister, such that their inherenttendency to bend, or twist is overcome. The transition from arectangular array to a triangular pitched array is more predictable andwell defined, as compared with earlier designs.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiment of the invention and the best mode for carryingout, will be described herein after with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of a prior art fuel consolidationstation, in which the preferred embodiment of the present invention isshown as the central station;

FIG. 2 is a perspective view the preferred embodiment of the invention,wherein the box portion of the fuel transfer canister have beenpartially cut away to show the grid structure at the top and the platemembers at the bottom;

FIG. 3 is a elevation front view of the transfer canister shown in FIG.2 with the front panel removed to show the upper, grid section and thelower, plate section;

FIG. 4 is a plan view of the transfer canister shown in FIG. 3, takenfrom above with the top plate removed to show the rows and columns ofcells formed by the grid structure;

FIG. 5 is a section view taken along lines 55, of FIG. 3, showing thereduced dimensions of the box and cells part-way down the box taper;

FIG. 6 is an elevation view of the canister of FIG. 3, taken from theleft side with the left panel removed to show the grid structure andplate structure therein;

FIG. 7 is a perspective view of the intersection of row layers andcolumn layers showing the short and long slits on individual gridsegments, and a skin surrounding and joining the layer outer edges;

FIG. 8(a) is an elevation view of the grid row layers of FIG. 7, withthe column layers oriented perpendicularly to the plane of the drawing,and FIG. 8(b) is an alternative embodiment with modified tabs on theouter edges;

FIG. 9 is a perspective view of a corrugated plate member showing theintegrally formed channels for guiding the fuel rods into a triangularpitch;

FIG. 10 is an end view of two adjacent corrugated edges of the platemembers, showing how the arrangement and spacing of the convex andconcave arches guides the fuel rods into a tightly packed triangularpitched array.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a portion of a nuclear fuel rod consolidation system 10 ofthe type generally disclosed in U.S. Ser. No. 535,105 "System and Methodfor Consolidating Spent Nuclear Fuel", the disclosure of which is herebyincorporated by reference. The portion of the system in which thepresent invention can be best described, comprises a disassembly station12, a fuel rod transfer station 14, and a fuel consolidation station 16.As more specifically described in the incorporated reference, a nuclearfuel assembly 18 is placed within the disassembly station 16 where theassembly structural members such as end fitting 20, are removed and theindividual fuel rods 22 are lifted out of the assembly. The fuel rodswill then be inserted into the transfer canister 24 where their spacingwill be significantly reduced, and the transfer canister 24 will belifted through the table 26, lowered onto the storage box 28, and thefuel rods lowered from the transfer canister 24 down into the storagebox 28, by means of first and second elevators 30, 32 respectively. Toimprove understanding of the operation of the consolidation system 10, acontainer spanning the distance between the fuel pool cavity floor 34and the table 26, of the type shown at 36 surrounding the fuel assembly18, have been omitted with respect to the transfer station 14 and theconsolidation station 16.

The present invention is directed to features associated with thetransfer canister 24. The canister according to the present invention,provides functions similar to those described in the incorporatedreference, but with significantly improved structure. In essence, thetransfer canister 24 receives at its upper end, a relatively looselypacked, rectangular array of fuel rods which are funneled through thecanister and rearranged into a close packed, triangular array resting onthe canister base 38 (shown in phantom). After the canister 24 has beenfilled and relocated on the table 26 above the storage box 28, thelocking means 40 are released so that the canister base 38 rests on thesecond elevator 32. By sequentially actuating the first elevator 30 andsecond elevator 32, the canister base 38, and the fuel rods supportedthereon, are lowered into the storage box in unison. The transfercanister 24 may then be reused in the consolidation system 10, and thestorage box 28 is removed from the consolidation system 10 and placedinto a nuclear fuel storage area (not shown).

The transfer canister 24 is a generally elongated, tapered box ofgenerally rectangular cross-sections having upper and lower internalsections 42 and 44, respectively. The upper section has grid means 46for receiving individual fuel rods in a relatively loosely packedrectangular array of rows and columns, and for urging the rods closertogether as they are lowered into the box, such that the rods emergethrough the lower end of the upper section, in a relatively tightlypacked rectangular array. The lower section 44 is adapted to receive therods as they emerge from the upper section. The rods pass through aplurality of substantially vertical plate members 48 corresponding tothe number of columns of fuel rods in the upper section. When the word"corresponding" is used in this sense it should be realized that"approximate corresponding" is meant since there is always one more opensquare in a column or row than there are intermediate plates between theopen squares, as seen, for example, in FIG. 4. The upper and lowersections 42, 44 will now be described in greater detail.

FIG. 2 is a perspective view of the preferred transfer canister 24, andthe end in which the box outer structure has been arbitrarily identifiedas comprising front panel 50, back panel 52, left side panel 54, andright side panel 56. Preferably, each panel is downwardly tapered suchthat the rectangular cross-sectional area at the upper end 58 isapproximately equal to the cross-sectional area of two nuclear fuelassemblies 18 (FIG. 1), and the rectangular, cross-sectional area of thelower end 60 is approximately equal to that of one fuel assembly.

At the open, upper end 58 of the canister 24 a top plate 62 providesguide means in the way of rows and columns of holes 63, through whichthe fuel rods are maintained in a predetermined, rectangular array asthey enter the canister. The top plate 62 is partially cut away to showthe uppermost extent of the grid means 46, which defines an individualcell 64 beneath each hole 63 in the top plate 62.

The lower portion of the canister 24 is cut away to show corrugatedplate members 48, disposed side-by-side between front panel 50 and backpanel 52. The lower edges 66 of the plates are corrugated and, in thepreferred embodiment, terminate at different elevations within thecanister 24.

FIGS. 3-6 illustrate in greater detail the relationship between thegrids 46 and the plates 48 within the canister 24. FIG. 3 is anelevation view of the canister 24 viewed from the front, with the frontpanel 50 removed. FIG. 4 is a plan view of the canister 24 from above,with the top plate 62 (FIG. 2) removed. The upper section 42 when thusviewed from above, has a multiplicity of honey-comb, substantiallycontinuous cells arranged in a rectangular array of, for example, twentyrows 68 extending from the left panel 54 to the right panel 56, andeighteen columns 70 extending from the front panel 50 to the back panel52. As the box tapers downwardly to the interface 72 between the upperand lower sections 42, 44 respectively, the cross-sectional area of eachcell varies with elevation in substantial proportion to the variation inthe cross-sectional area of the canister or box 24. FIG. 5, across-section taken at line 5--5, illustrates the proportional decreasein the area of cell 64 as the dimensions of the panels 50, 52, 54 and 56have decreased. In the preferred embodiment, wherein the upper section42 is approximately 100 inches in length, the cross-sectional area atinterface 72 is approximately 60% of the cross-sectional area at the topof the canister 24.

Fuel rods inserted through the upper section 42 emerge into the lowersection 44 in a relatively tightly packed, rectangular array. The lowersection 44 is adapted to receive the rods within a plurality ofsubstantially vertical plate members 48, the number of which correspondto the number of columns 70 of fuel rods in the upper section 42. Theseplates 48 are shaped and aligned such that the rods passing between themare forced into a closely packed, triangular array at the lower end 60of the canister 24. The following sections provide additional detail onthe inventive features associated with the grid means 46 constitutingthe upper section 42, and the corrugated plate members 48 constitutingthe lower section 44.

GRID STRUCTURE

The description of the grid structure will proceed with reference toFIGS. 3 through FIG. 6. FIG. 6 is an elevation side view of the canister24 shown in FIG. 3, wherein the side panel 54 has been removed. Theupper section 42 comprises a plurality of vertically overlapping gridlayers 74a-74i and 76a-74h. Each grid layer 74a-74i comprises aplurality of grid segments 78 which extend from the front panel 50 tothe back panel 52 and will be conveniently referred to as columnsegments constituting column grid layers. A plurality of transverselyoriented segments 80 likewise will be referred to as row segments, andconstitute the row grids 76.

It may be seen that column grid layers 74e and 74f have interposedbetween them row grid layer 76e. The overlapping intersection of thecolumn layers 74 and row layers 76 provide alternating row and columnstructure which forms a substantially continuous honey-comb oflongitudinal cells 64. To accommodate the overlapping or stagering ofthe transverse layers, the uppermost and lowermost column grid layers74a and 74i, are half the height of the other column and row grids 74,76. Typically, the entire grid structure 42 effectively contains eightcolumn grids 74 and eight row grids 76.

In FIG. 6, it may be seen that row layers 76g and 76h may have less thana full complement of row segments 80 as a way to minimize the spaceoccupied by structural material between converging fuel rods.

FIG. 7 illustrates in detail, the intersection of, for example, row grid76d, column 74e and row grid 76e. FIG. 7 is a perspective view orientedsimilarly to FIG. 2. FIG. 8 is an elevation view facing the row grids76d and 76e as shown in FIG. 7. In FIG. 7, the tapered sides of thesegments and layers are not illustrated since the focus is on theintersection of the layers. In FIGS. 7 and 8, the layers and segmentshave been simplified with respect to the number of cells, relative toFIG. 3-6, for clarity of illustration.

Referring now to FIGS. 7 and 8, each layer 76d, 74e, and 76e, comprisesa plurality of substantially parallel, segments 78, 78' and 80, 80'.Each segment has a top edge 82, a bottom edge 84 and side edges 86. Theside edges 86 of each segment are tapered to conform to the taperedpanels 54, 56 (or in the case of column segments 78, to front and backpanels 50, 52), at the elevation within the canister 24 where thesegments are to be located. The outer dimensions of row segments 80 inlayer 76 preferably, the bottom edge 84 of segment 80 is longer than thebottom edge 84' on segment 80', but is substantially the same length asthe top edge 82' of segment 80'. Although the top and bottom edges 82,84 are not the same length, they are parallel and thus the segment istrapezoidal. Each edge 82, 84 has alternating short and long slits, 94,96, respectively, by which the segments are interconnected.

When the entire grid structure is assemblied within the canister 24 themultiplicity of cells defined by the intersection of the segments 78, 80each provides a funnel for the individual rod inserted therein. Thefunnel results from providing the slits along each edge, such that thedistance between slits is proportional to the length of the edge. Thus,each succeeding vertical portion of a given cell, has a slightlydifferent dimension due to the proportionate spacing of the slits.

In the preferred embodiment, the combined length of a short and longslit which are in substantial vertical alignment, is approximately equalto one half the distance between the top and bottom edges of thesegment. This assures that each cell wall will be substantiallycontinuous and thereby avoids the possibility that fuel rods may hangupbetween segments of adjacent layers. Thus, it is preferred that theupper edge of segment in a row layer is substantially in contact 90 withthe lower edge of a segment in the next higher row layer, and likewisefor the column layers.

The arrangement of long and short slits permits ready construction ofthe grid layers and interconnections by transversely securing the longslits of one segment with the short slits of another segment. Thetransversely adjacent segments in a given row or column layer, such as78a, 78b, and 80a, 80b in FIG. 7, alternate between, for example, the"four short, three long" slotted upper edge 78a and 80a and the "fourlong, three short" upper edge of segments 80b and 78b. The lower edgeslot pattern likewise alternates. The vertically adjacent segments ofdifferent row and layers, e.g., segments 80 and 80' in row layers 76d,76e, respectively, have similar slit patterns. This arrangement ofsegments having long and short slits assures that any portion of asegment serving as a cell wall, has only one long slit boundary. Thus,the portion remains more rigid and flatter, than if bounded by two longslits.

It should be appreciated that the grid structure 46 is preferablyassembled in its entirety, before insertion into the canister 24. Thesegments of each layer are interconnected starting with the lowermost74i and 76h and the segments may be tack welded together to establishsufficient rigidity so that the structure may be handled for insertioninto the canister. One such welding technique includes welding a ribbonor strip around the perimeter of the structure, at the interface betweenadjacent row layers, and likewise between adjacent column layers. Oneassembly technique is illustrated in FIG. 7 and 8(a), wherein a shroudor skin 100 with perforations 102, has slightly smaller dimensions thanthe tapered box 24 (FIG. 2), and is installed over tabs 104 on theperimeter or outer edges 86 of the layers. The tabs are spot welded andthe entire shrouded grid section 42 is then lowered into the box. Whenthe grid section is used in conjunction with the plate section 44discussed below, the shroud or skin 100 spans the entire box and helpsconnect the grid section to the plate section.

FIG. 8(b) shows a preferred embodiment wherein the tabs 104' areextensions at the lower 84 and upper 82 edges of each segment so thatthe tabs from vertically adjacent segments penetrate the same shroudperforation 102 and can be joined with a single weld 106. The portionsof segments near the box wall, that are otherwise unsupported due to abordering long slit, can thus be strengthened. layers.

PLATE MEMBERS

The detailed description of the lower section 44 having the platemembers 48, will proceed with reference to FIGS. 3, 6, 9 and 10, whereinspecific structure will identified with numerals beginnning with 200.

FIG. 9 shows a plate member 48 having substantially parallel upper andlower edges 200, 202 respectively, and tapered side edges 204. The sideedges 204 are tapered to conform to the front and back panels 50, 52 ofthe lower portion of the canister 24. Each plate 48 has a plurality ofcorrugated channels 206 corresponding to the number of rows 68 of fuelrods (see FIG. 4). The corrugated channels 206 extend to the lower edge202 and form a periodic series of alternating convex and concave arches208, 210 respectively. Preferably, each plate 48 has a substantiallyflat area 212 beginning at the upper edge 200, with a gradual transitionsuch that the amplitude of the convex and concave arches 208, 210,increase with distance from the upper edge 200.

The plate 48 is preferably formed in a die that has the required numberof "dummy" rods welded to one die plate, each rod tapering slightlytowards the center of the plate to converge the corrugations and inaccordance with the overall designed taper of the side edges 204. Priorto pressing, however, the plate is rectangular and the side edges 204will be finely dimensioned for mating with the canister panels, afterpressing in the die. The integral formation of the corrugations havingincreasing amplitude as they approach the lower edge 202, tends to drawthe side edges inward during pressing, but this is insufficient toproduce the desired end taper on the side edges 204. The transition fromflat to full amplitude on the corrugations is accomplished in the die byorienting the movable die member at a slight angle relative to thestationary die on which the dummy rods are secured, so that fulldeformation of the plate occurs only near the lower edge 202.

In the preferred embodiment, the upper end edge 200 of each plate isadapted to mate with and form a continuous guide path from, the gridmeans 406 of the upper section 42.

With reference to FIGS. 3 and 6, it may be seen that this transitionoccurs at the section interface 72. The simplest way to accomplish thisinterface is to provide an unslotted edge at the upper edge 200 of eachplate 48 so that these engage long slits 96 provided on the lowermostsegments 76h, of the grid means 46 (see also FIG. 8). At the interface72, a band 216 or other member may be tack welded around the peripheryof the interconnected structure, as was suggested with respect to theassembly of the layers 74, 76 of the grid structure 46, or in thepreferred mode, the plates have tabs to mate with shroud perforationsand for welding to tabs on the segments of grid layer 74i.

In the preferred embodiment, the plates are not all of the same length,so that the lower edges 202 are staggered. In FIG. 3, the lower edge202a of the central plate and the plates between the center and the leftpanel 54, are labeled 202a-i respectively. When viewed from the side ofcanister 24 with left panel 54 removed, a different view of thestaggered edges can be seen.

This staggering is necessary to maximize the consolidation within thecanister since the plates have a significant thickness relative to afuel rod diameter and the fuel rods should ideally have no separationwhen grouped in the triangular array at the lower end 60 of the canister24.

FIG. 10 shows the relationship of, for example, plate lower edge 202aand adjacent 202b. Each convex arch 208 on each plate 202a faces aconcave arch 210 on plate 202b, and each concave arch 210' on plate 202afaces a convex arch 208, on 202b. The distance 218 between each convexand facing convex arch is substantially equal to a fuel rod diameter, asshown in FIG. 10, in phantom. This alignment and spacing of the platescauses the leading ends of the tightly packed rectangular array of fuelrods received by the lower section 44 to be consolidated between theplates and the box panels into a tightly packed triangular array at thelower edges of the plates. FIG. 10 also clarifies the meaning of archamplitude, where the convex amplitude 220 and concave amplitude 222 aremeasured relative to a flat imaginary plane 224 an equal distancebetween them.

I claim:
 1. A nuclear fuel rod consolidation canister for theconsolidation of nuclear fuel rods, comprising:an elongated, tapered boxof generally rectangular cross-section having upper and lower internalsections; the upper section having grid means for receiving individualfuel rods in a relatively loosely packed rectangular array of rows andcolumns, and for urging the rods closer together as they are loweredinto the box, such that the rods emerge through the lower end of theupper section in a relatively tightly packed rectangular array; thelower section being adapted to receive the rods as they emerge from theupper section, and having a plurality of substantially vertical platemembers corresponding to the number of columns of fuel rods in saidupper section; each plate having substantially parallel upper and loweredges, inwardly tapered side edges abutting the box, and a plurality ofintegrally formed, corrugated channels corresponding to the number ofrows of fuel rods, the corrugations extending to the lower edges of eachplate and forming a periodic series of converging alternating convex andconcave arches; wherein each plate is aligned with an adjacent platesuch that each convex arch on one plate faces a concave arch on anadjacent plate, and each concave arch on said one plate faces a convexarch on said adjacent plate; whereby the leading ends of the tightlypacked rectangular array of fuel rods received by the lower section areconsolidated between the plates and box, into a tightly packedtriangular array at the lower edges of the plates.
 2. The canister ofclaim 1 wherein each corrugated plate is substantially flat at the edgelocated nearest said upper section, and wherein the amplitude of theconcave and convex arches increases with the distance away from saidupper edge.
 3. The canister of claim 2 wherein the upper edge of theplate members is adapted to mate with and form a continuous guide pathfrom, the grid means of the upper section.
 4. The canister of claim 2wherein the distance between the upper edges of adjacent plate membersis substantially equal to the distance between fuel rod centers in thetightly packed rectangular array at the lower end of the upper section.5. The canister of claim 1 wherein the lower edges of each plate isspaced from an adjacent plate such that the distance between each convexand facing concave arch is substantially equal to a fuel rod diameter.6. The canister of claim 1 wherein the lower edge of at least some ofthe plates, are vertically staggered.